In the conventional petroleum industry it is well known in the art how to find hidden underground reservoirs, how to drill and complete wells in the petroleum reservoir, and how to withdraw petroleum products to economic depletion and the like. In conventional petroleum reservoirs the petroleum itself is quite mobile and often will flow to the surface once a well is drilled into the reservoir. The reservoir exists because petroleum migrated into the permeability and porosity of the host rock (usually of sedimentary origin) and was trapped in place by a fault, permeability pinchout and the like. The petroleum under these circumstances is flowable and will move under the influence of differential pressure. Such a petroleum would contain an array of hydrocarbons, varying from those that would be gases at atmospheric pressure to those that would be quite viscous were they not mixed with lighter constituents.
A special case of petroleum reservoirs are those commonly called heavy oils, bituminous deposits and tar sands. These are more properly termed deposits because the petroleum values cannot be recovered by conventional oil field techniques. These deposits are for all practical purposes composed of petroleum hydrocarbons that are immobile. Immobility is occasioned by the fact that all or virtually all of the lighter fractions of conventional petroleum crude are missing, leaving a residue of viscous heavy hydrocarbons. In some case these heavy hydrocarbons have the appearance of hard solid substances that fill the void space in the host rock in somewhat the same manner as grout. In other cases these heavy hydrocarbons have the appearance of a sticky semi-solid that has adhesive qualities somewhat like that of glue. In either case it is virtually impossible and certainly impractical to attempt to dislodge the heavy hydrocarbons by applying differential pressure.
All of the petroleum hydrocarbons in these heavy hydrocarbon deposits have one thing in common. Upon application of heat the heavy hydrocarbon mixture becomes semi-liquid, then liquid, then free flowing liquid. It is not uncommon to find a heavy hydrocarbon mixture that at a temperature of 200.degree. F has the flow characteristics similar to a conventional quality petroleum crude at room temperature. Fortunately most heavy oil crudes attain acceptable fluidity at temperatures well below ignition temperatures so there is no danger of the crude flashing to fire upon being exposed to air.
The problem of production of heavy oil crudes, as is well known in the art, is how to bring the massive deposit up to flowable temperature so that it can be produced as a petroleum reservoir. The problem is further complicated by the fact that it is quite common to find the deposit being composed of a series of lenticular segments, which probably are interconnected. Generally these interconnecting passages between the lenticular segments are so small compared to the individual segment that it is very difficult to transfer heat from one segment to the next. Thus each segment tends to be a deposit or reservoir to itself.
There have been numerous schemes advanced for the addition of heat to tar sands and similar deposits, from underground nuclear explosions to setting the deposit afire in the manner of conventional petroleum fire floods. Other schemes have fostered the idea of heating the deposit by heat transfer from circulating fluids. All schemes tried have had a measure of technical success, but have failed as commercial ventures as a general rule. One notable success is a project in the Athabaska tar sands of Canada wherein heat is not applied to the deposit, but rather the deposit is grubbed up and transported to an above ground processing plant.
There are numerous reasons why methods of the prior art have been failures in the commercial sense in trying to add heat to the deposit. First, the heavy hydrocarbons themselves are poor conductors of heat, thus the scheme to use heated hydrocarbons to transfer heat to cooler hydrocarbons is a slow process with heat losses to surrounding host rock tending to negate the positive effects. The host rock is also a poor conductor of heat which thwarts efforts to speed up heat transfer to the heavy hydrocarbons, and further serves as a heat sink to diminish the efficiency of applying heat for a useful purpose. The host rock filled with grout-like heavy hydrocarbons has a very low effective permeability to permit the invasion of hot gases or vapors such as steam. This low effective permeability prohibits the use of heat and pressure to drive the heavy hydrocarbons to an adjacent production well because the heavy hydrocarbons mobilized by the heat will be driven by pressure into the available permeability, become cooled and immobile away from the influence of the heat, and thus effectively plug all the remaining permeability.
Thus it is apparent that removal of mobilized heavy hydrocarbons should be in the direction of the oncoming heat, which is to say that the residual mobilized petroleum must move generally countercurrent to the flow of heat. Once this mechanism is established the flow of petroleum will be facilitated by becoming warmer and less viscous. Further, by withdrawing petroleum from its previously locked position, effective permeability is significantly increased through the host rock, permitting the application of heat from the carrier fluid directly to exposed residual heavy oil crude to be heated, mobilized, withdrawn, and so on.
It is an object of the present invention to disclose new methods of mobilizing viscous residual hydrocarbons by the application of heat by means of a carrier fluid, withdrawing the mobilized heavy hydrocarbons to an underground location and conveying the hydrocarbons to the surface of the ground. Other objects, advantages, and capabilities of the present invention will become apparent as the discription proceeds and in conjunction with the accompanying drawings.